The present invention relates to power supplies for battery powered devices. It finds particular application to situations in which it is desirable to supplement the energy provided by or otherwise charge the main battery of a battery powered device with energy from an auxiliary battery.
Recent years have seen a proliferation of battery powered devices. Digital cameras, personal digital assistants (PDAs), hand held games, portable audio players, remote controls, wireless computer keyboards and mice, portable and remote data communications devices, and mobile telephones are but a few examples.
These devices typically include one or more main batteries which power the device during normal operation. Where the main batteries are rechargeable (secondary), the devices are sometimes provided with battery charging circuitry which provides electrical energy for charging the batteries. Depending on the implementation, the charging circuitry may be located either internal or external to the device itself. In any case, the charging circuitry is ordinarily powered from an alternating current (AC) power outlet, a 12 volt direct current (VDC) automobile power outlet, or the like. Consequently, the device must be returned to a fixed or semi-fixed location for charging.
So-called hybrid battery management technology, which uses an external auxiliary battery together with associated charge control circuitry, has been used to provide supplemental power to battery powered devices. In one implementation, the auxiliary battery and charge control circuitry have been housed in a generally cylindrical housing, which has been suspended at one end of a cable, with the other end connected to the positive and negative terminals of the charging or power port of the battery powered device.
FIG. 1 shows the output current Iout provided to a device having a lithium ion (Li-ion) main battery according to a hybrid battery management technique. Where the main battery voltage is less than a first value, the output current is substantially independent of the main battery voltage. Where the main battery voltage has a relatively higher value, the output current is a substantially linear function of the main battery voltage. Where main battery voltage has had a still higher value, the output current has again been substantially independent of the main battery voltage. As can also be seen in FIG. 1, the output current has also been a function of the auxiliary battery voltage.
While these techniques have provided for more convenient charging of the device's main battery during mobile operation, there remains room for improvement. In particular, it remains desirable to extend the operating life of the auxiliary battery while minimizing modifications to the battery powered device. In many cases, it is also desirable that the external device have a form factor which complements that of the battery powered device.